Shuts off when downshifting

I have the truck listed above it is the XLT model. Just recently the transmission sounds like it is dragging when downshifting coming to a stop. Then the vehicle will shut off. This only happens when slowing down to come to a stop. I took it to a mechanic today. He told me it was the torque converter switch inside the transmission. He has not called me back on cost and labor yet. I was wondering if anyone else has had this problem and what they did to fix this problem. Thanks